The present invention relates to a method for dry etching of a substrate surface or, more particularly, to a method for pattern-wise etching of a surface layer of, for example, elementary silicon or a compound of silicon formed on a substrate in a dry process by exposing the surface to an atmosphere of plasma.
As is known, manufacturing of various kinds of electronic devices such as ICs and LSIs includes a process of pattern-wise etching of a substrate surface or a surface layer formed on a substrate and the etching is performed either in a wet process using an etching solution or in a dry process by the techniques of, for example, plasma etching, sputtering etching, reactive ion etching (RIE), ion beam etching and the like. These dry process methods for etching are preferred to the wet process by virtue of the better precision for fine working and controllability as well as higher possibility of the process for simplification and automatization and less problems of environmental pollution.
On the other hand, the methods of dry etching are usually disadvantageous due to the expensiveness of the apparatus used in the method. In addition, problems are encountered in the selection of the etching gas when the method is industrially practiced. For example, use of a highly reactive gas capable of giving a high etching rate usually increases the undercutting or side etching behind the patterned areas of the resist layer so that the pattern formed by use of such an etching gas cannot be fine enough as desired while the use of an etching gas to cause less side etching necessarily takes a long time for the etching treatment due to the lower etching rate. Thus, the requirements for smaller side etching and higher etching velocity are incompatible with each other.
In particular, the dry etchings of a silicon compound or polysilicon which is important in the preparation of LSIs by the plasma etching is usually performed by use of a gaseous mixture of carbon tetrafluoride containing a few % by volume of oxygen as the plasma gas. This etching gas is currently most widely used due to the high etching rate of 300 to 600 nm/minute as well as due to the advantages in the selectivity, handleableness and safety. The etching proceeding by use of this mixed etching gas, however, is isotropic so that the width of a pattern formed by etching is usually smaller than the width of the pattern on the photomask through which the photoresist layer has been exposed to light and no etched pattern of a width of 3 .mu.m or smaller can hardly be obtained by use of such an etching gas. In other words, precision in pattern-wise etching is an essential requirement in a high-precision fine working in compliance with the trend toward higher density and higher performance of LSIs and the like electronic devices which can be performed only by use of an etching gas having anisotropy or directionality in etching.
There is, however, usually a contradictory relationship between the anisotropy of etching and the etching rate. For example, sulfur hexafluoride gas cannot be used as an etching gas for fine patterning due to the large side etching resulting only in an unsatisfactory precision of patterning despite the high etching rate thereof. On the other hand, anisotropic etching with small side etching can be performed by use of a gaseous mixture of chlorine with carbon tetrachloride, pentafluorochloroethane or hexafluoroethane as the etching gas but the etching rate of these etching gases is as low as 100 to 200 nm/minute taking an unduly extended etching time so that certain disadvantages are unavoidable including the decrease in the thickness of the resist layer and poor selectivity in the etching relative to the base surface. At any rate, the requirements for the anisotropic etching and the higher etching rate are incompatible with each other and no etching gas for plasma etching is known in the prior art which satisfies both of these requirements.